The specification of International patent application No PCT/AU93/00453 discloses a range of radiation control devices (including radiation deflectors, concentrators and lenses), each of which is formed by an array of substantially identical reflecting elements. In addition, it is noted in the specification of International patent application No PCT/AU93/00453 that an array of elongate reflectors can be used for one-dimensional focusing (line focusing) of radiation, and that two such arrays of reflectors, crossed at right angles, will form the equivalent of an array of reflecting channels, with each channel having a rectangular cross-section (which is a square cross-section when the reflectors of each array of parallel reflectors have the same spacing). Thus a two-dimensional focusing array or lens can be constructed using two arrays of reflector strips or slats.
One advantage possessed by optical devices formed by an array of reflecting slats--including the "90.degree. crossed-slats" lens referred to above--is that the surfaces of the elongate reflectors can be polished and examined for flaws before the arrays of reflectors are assembled to form the optical device. This feature, as also noted in the specification of International patent application No PCT/AU93/00453, permits the reflective surfaces of the slats to be coated in a manner which ensures that those surfaces are selectively reflective at predetermined wavelengths.
The present inventor has also shown that this type of 90.degree. crossed-slats lens has an additional advantage over the devices previously used to focus x-radiation (and other radiation having a wave-like character) onto a focal zone. The additional advantage is a significant increase in the "intensity gain" of the lens or focusing device. In this field, the gain, G. of a lens or focusing device is defined by the relationship: ##EQU1## where I.sub.d is the intensity of radiation in the focal spot or zone of the device and I.sub.o is the intensity of radiation at the same location in the absence of the focusing device.